Re: Tek 494AP VR board mystery


John Miles
 

I'm almost 100% sure it is just a snafu that happens all the time and was
quite widespread back then when CAD didn't exist yet but leave 0.00001%
doubt of missing something.
Agreed, that is very weird. Same with those resistors... usually you try to
minimize the number of BOM line items by consolidating resistors, but they
seem to have done the opposite here.

The LC BPF at the output of the A69 post-VR amp is also funky. On the
schematic, the inductors are 0.652 uH, 0.568 uH, 0.652 uH, 0.94 uH, 1.047
uH, 1.047 uH, and 0.94 uH, but in the parts list they are more
reasonable-looking values. Meanwhile, the capacitors are bog-standard 220
pF and 470 pF parts.

The filter could be explained by the engineering department getting a little
too carried away with their new computer, or maybe they just measured the
parts they used and transcribed the values to the BOM without rounding.
Likely true for the "1.047 uH" toroids, but it doesn't explain the others,
and nothing explains CR3068 as far as I can tell. I wonder if it's an IP
protection feature. If Hickok or Lavoie or somebody like that were to clone
the instrument, things would get awkward if they had to explain what that
diode was for in court. Similar to the practice of adding fictitious towns
to a map to guard against commercial copying.

-- john, KE5FX

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